1. Field of the Invention
The present invention is related to a semiconductor device and a manufacturing method thereof. More specifically, the present invention is directed to a coupling conductor which connects a semiconductor element to a lead terminal, for instance, such a coupling conductor which electrically connects a source electrode of an MOSFET corresponding to one of semiconductor elements with a lead terminal thereof.
2. Description of the Related Art
Very recently, reductions of power consumption of semiconductor devices have been desired. As one of means for reducing the power consumption, various technical ideas capable of lowering ON resistances have been proposed, while the ON resistances correspond to essential resistance components of semiconductor devices when the semiconductor devices are operated. Among these technical ideas, a description is made of techniques capable of reducing ON resistances occupied by packages of semiconductor devices, while these packages protect semiconductor elements and play a role as external connection terminals.
Among manufacturing steps of semiconductor devices, one example of assembling steps is performed as follows: That is, firstly, a semiconductor element cut out from a wafer where desirable element regions and wiring lines have been formed is mounted on an island portion of a lead frame which has been equipped with an island portion (namely, semiconductor element mounting portion) and a lead portion. The lead portion is constructed of a lead terminal manufactured in such a manner that a tip portion of the lead terminal is located in the vicinity of this island portion. Next, an element electrode formed on a surface of the semiconductor element is electrically connected to the lead terminal provided in the vicinity of the circumferential portion of the island portion by employing a coupling conductor such as gold wires and aluminum wires. Thereafter, both the semiconductor element and the lead frame are sealed by a resin, or the like so as to be manufactured in a package form, resulting in a semiconductor device, while a portion of the tip portion of the lead terminal is left. In this case, it is so assumed that a package implies such a component made by combining a lead frame containing a lead with a sealing resin. FIG. 8 is a diagram for representing a semiconductor device of the conventional technique.
In this case, when a coupling conductor called as a bonding wire and made of gold wires, or aluminium wires is employed so as to connect the semiconductor element to the lead, a diameter of a single wire is approximately several tens μm to several hundreds μm. In order to reduce ON resistances, several tens to several hundreds of either gold wires or aluminium wires are necessarily employed, which may cause cost-up aspects, and assembling steps to become complex.
As a consequence, a patent publication 1 has described such a method that a plate-shaped coupling conductor made of copper is employed instead of the metal wires so as to electrically connect a semiconductor element to a lead terminal. In order to electrically connect an element electrode to the lead terminal by employing the coupling conductor, a conductor such as soldering is employed.
Also, a patent publication 2 has described such a method that a plate-shaped coupling conductor made of aluminium is employed so as to electrically connect a semiconductor element to a lead terminal. In order that an element electrode is electrically connected to the lead terminal by the coupling conductor, these electronic components can be connected to each other by performing an ultrasonic connecting method without employing soldering and the like.
Since the plate-shaped coupling conductor made of either copper or aluminium is employed, the ON resistance occupied by the package of the semiconductor device can be reduced, so that such a resistance value lower than, or equal to 1 milliohms can be realized. Moreover, in view of heat radiation characteristics, since thermal conductivity of the above-described plate-shaped coupling conductor becomes higher than that of the gold wires, or the aluminium wires, the heat radiation characteristic from the semiconductor element to the lead frame may be improved, so that a higher current capacity may be realized.
Patent Publication 1: JP-A-8-148623 (page 4, FIG. 1)
Patent Publication 2: JP-A-2002-314018 (page 16, FIG. 2)
However, in such a case that the plate-shaped coupling conductor made of copper is employed, in order to electrically connect the element electrode to the lead terminal, these electronic components must be connected to each other by employing such a conductor as solder. Also, in order that a metal material having a superior solder accepting characteristic must be employed as an element electrode, there is a problem that aluminium which is generally employed as a material of an element electrode cannot be utilized.
On the other hand, although electric conductivity of aluminium is a second higher grade subsequent to copper, this electric conductivity of aluminium is lower than the electric conductivity of copper by approximately 60%. Accordingly, such an aluminium electrode having a plate shape whose thickness is approximately 2 times thicker than a thickness of a copper electrode must be employed. As a result, there is such a demerit that the thickness of the package of the semiconductor device becomes thicker. In addition, since the thermal conductivity of aluminium is lower than that of copper by approximately 60%, there is another demerit that thermal conductivity from the semiconductor element to the lead frame is lowered, as compared with that of copper.
As a consequence, an object of the present invention is to provide a semiconductor device having a coupling conductor, while the coupling conductor is equipped with a function capable of achieving both electric conductivity and thermal conductivity owned by copper, and also, another function capable of simplifying an electric connection between an element electrode and a lead terminal, which is owned by aluminium.